To print effectively and efficiently using a color thermal printing process, the dye impregnated color donor web must be properly positioned relative to the dye receiver. Proper positioning is required to ensure full coverage of the image area by successive color patches. The typical color donor web contains a repeating series of yellow, magenta and cyan color patches, and in some cases, a black patch and/or a clear fusing patch. Each patch must be properly aligned with the receiver to ensure high quality printing. One way to align or index the receiver and donor is by using a detector which will detect whether the color is yellow, magenta, cyan, black or clear, and identify its position.
One type of sensing system is disclosed in U.S. Pat. No. 4,551,729 which issued Nov. 5, 1985 to Takeshi Kubo, Junji Kawano, Fumio Takahashi and Tsutomu Yamaguchi, wherein a donor web is encoded with marks that are detected by a detecting means after the donor passes the thermal print head. This system, however, is used with paper in the fanfold design with pre-punched or pre-drilled edges. The drum has a large diameter so that the indexing of the donor web with the paper is not absolutely critical but there must be general alignment. The general alignment is obtained with the sensor elements placed in the donor path past the thermal print head so that the marks are detected after the donor emerges from the print head. One of the difficulties with this system is that the pre-punched holes, which the protrusions on the drum engage to advance the donor and the receiver, are prone to stretching and misalignment, thereby degrading the print quality.
A thermal printer with a different sensing arrangement is disclosed in U.S. Pat. No. 4,620,199 which issued Oct. 28, 1986 to Hisao Tatsumi, Haruhiko Kayata and Fumio Watanabe. The donor web is in a cassette with an opening therein for engagement with a sensor. The donor passes by the sensor at one point which is relatively far from the area where printing occurs. The color sensor senses the color of the donor as the donor is unwound from the donor supply spool before printing occurs. Understandably, much could happen between the location of the sensor and the printing location while the donor ribbon traverses this course. Accordingly, it will be appreciated that it would be highly desirable to have a color sensor to accurately sense the position and color of the donor ribbon for accurate registration of the colors during printing.
U.S. Pat. No. 4,710,781 which issued Dec. 1, 1987 to Stanley W. Stephenson discloses an apparatus for identifying different color frames of a donor web. A sensor includes a light emitting diode (LED) to emit red or yellow light and a corresponding photodetector to respond to the red or yellow light. A space saving arrangement positions two LEDs to illuminate the same spot on the donor web adjacent an edge of the web. The yellow and red light pass through dye frames of the moving donor web and illuminate the appropriate photodetectors. The general alignment is obtained with the sensor elements placed in the donor path past the thermal print head so that color frames are detected after the donor emerges from the print head. Where frames are detected after printing, there is an amount of each frame, equal to the length of donor between the print head and detectors, that is wasted. Accordingly, it will be appreciated that it would be highly desirable to have sensors to accurately sense the position and color of the donor web for accurate registration of the colors during printing and to minimize wasted donor.